The present invention relates to a control device of a spark-ignition engine.
Prior art approaches for improving both an exhaust emission performance and a thermal efficiency are known. For example, JP2007-154859A discloses using a combustion mode of compressing to ignite mixture gas inside a cylinder. However, compression-ignition combustion occurs with a significant increase in pressure as the engine load increases, causing an increase in combustion noises. Thus, as disclosed in JP2007-154859A, even with engines which perform compression-ignition combustion, within an operating range on a high engine load side, instead of compression-ignition combustion, spark-ignition combustion by operation of an ignition plug has generally been performed.
JP2009-197740A discloses an engine which performs compression-ignition combustion within a low engine load operating range with low engine speed similarly to the engine disclosed in JP2007-154859A. With the engine, within the compression-ignition combustion performing range, open periods of intake and exhaust valves are adjusted to leave burned gas at a high temperature inside a cylinder so that the in-cylinder temperature is increased to stimulate the compression-ignition combustion. Whereas, within a part of the compression-ignition combustion performing range where the engine load is relatively high and the engine speed is relatively high, the open timing of an intake valve is advanced so that burned gas inside the cylinder is blown back to the intake port side once and then introduced into the cylinder again along with fresh air. In this manner, the temperature of the burned gas decreases because of the fresh air, and thus, within the relatively high-engine-speed high-engine-load range where the temperature and the pressure at the end of compression stroke become high, the significant pressure increase due to the compression-ignition combustion can be suppressed.
Meanwhile, in spark-ignition combustion, since thermal efficiency is relatively low, the combusting gas temperature increases. Whereas, in compression-ignition combustion, as described in JP2007-154859A and JP2009-197740A, the high-temperature burned gas is introduced into the cylinder to secure the ignitability. Therefore, with engines in which the combustion mode is switched according to the engine operating state, a temperature environment inside the cylinder is comparatively high and the high-temperature burned gas produced by the spark-ignition combustion is introduced into the cylinder immediately after the spark-ignition combustion is switched to the compression-ignition combustion, resulting in an excessive increase in the in-cylinder temperature. This excessive increase may cause pre-ignition such that the mixture gas within the cylinder is compressed to ignite in, for example, a compression-stroke period, and a pressure increase rate (dP/dθ) inside the cylinder may become significantly high to cause loud combustion noises.
Note that switching from spark-ignition combustion to compression-ignition combustion is not limited to be performed according to the engine load decrease, but may also be performed while the engine load is stable, as well as when an engine temperature increases from a cold-start state to a warmed-up state and under other circumstances.